Macronutrient selection of free-ranging urban Australian white ibis (Threskiornis moluccus)

Abstract

Anthropogenic habitats often provide urban wildlife the opportunity to feed on a range of nutritionally diverse foods, which may ultimately lead to human-wildlife conflict. The Australian white ibis (Threskiornis moluccus) provides an exemplar model for examining the nutritional priorities and constraints of a native vertebrate that is successfully transitioning to an urban specialist. Here, we used field-based feeding trials to investigate the macronutrient preferences of free-ranging ibis in Sydney, Australia. Feeding trials (n = 61) offering three experimental feeds showed that ibis selected significantly more high-carbohydrate (HC) than high-protein (HP) and high-lipid (HL) foods (95% CIHP-HC = -1.115 to -0.709; CIHL-HC = -1.874 to -1.468), and significantly more HP than HL (CIHL-HP = -0.962 to -0.556). The average proportion of macronutrient-derived energy selected by ibis was 25% protein (P; ± 1.1 SE): 23% lipid (L; ± 1.1): 52% carbohydrate (C; ± 1.8). Nutritional geometry suggested that mixtures selected at experimental feeders were substantially higher in C and lower in P than are natural prey (e.g. insects, crustaceans), which were composed primarily of P and L. Compositional log-ratio-based linear models of factors affecting macronutrient proportions selected by ibis showed that: 1) ln(P/C) increased with amount of recent rainfall; and 2) ln(L/C) also increased with rain and had a non-linear relationship with number of birds feeding. Our results suggest that ibis forage for macronutrients rather than energy "per se", and that their urban foraging is influenced by competition and the environment.